The present invention relates to a high temperature polymer resin for use as a stiffening agent for a filtering medium. More specifically, a polyimide treated pleated filter medium for use in a high temperature industrial particulate filter apparatus is disclosed.
A technique for controlling pollutants and emissions from industrial plants is to remove undesirable particulate matter carried in a gas stream by fabric filtration. Such fabric filtration is accomplished in a particulate matter or dust collection apparatus known in the industry as a xe2x80x9cbaghousexe2x80x9d. The fabric filter is preferably pleated to increase the effective filtering area while occupying the same or less space. The improved baghouse illustrated herein includes two large chambers, or plenums, that are divided by a tube sheet having a plurality of openings for receiving air filter cartridges.
The filter cartridges include a pleated filter medium that retains the unwanted particles as the air is forced through the cartridge. Traditional dust filtration materials are made from woven or needlepunch media. Newer filters, including higher surface area pleated media, are made from spunbond or other nonwoven media. The effectiveness of the filters diminishes as particles collect on the outer surfaces of the filters, thereby diminishing air flow. To remove accumulated particles on the filters without physically removing the filters from the baghouse, air may be pulsed through the baghouse cartridge filters from the opposite direction of air flow during particulate removal. The reverse pulses abruptly and temporarily expand the filter media to dislodge the particles, which fall to the bottom of the baghouse and are removed. The effectiveness of the baghouse is greatly increased without having to repeatedly remove and replace the cartridges
Unfortunately, there are limitations associated with the use of pleated filter media. First, the pleated structure is sensitive to temperature. Complications arise when these media are being processed and manufactured at temperatures similar to the temperatures used in the baghouse during filtering applications. The higher temperature causes the media to soften, allowing a level of pleat collapse or pleat pinch to occur. Pleat collapse can restrict air flow and cause increased pressure drop minimizing the advantages of the higher filter surface area of the pleats. For this reason, the use of polymer filter media for use at higher temperatures has been limited. (For purposes of this disclosure, high temperature is meant to include but is not limited to filtering applications ranging from about 300xc2x0 to about 500xc2x0 F., with surge temperatures typically reaching about 550xc2x0 F.)
There are several primary fibers or polymers, processed into fabrics and used in the 300xc2x0 to 550xc2x0 F. range. Traditionally they are polyphenylene sulphide (PPS, with trade names such as Ryton(copyright), Torcon(copyright) and Procon(copyright)) and aramid (Nomex(copyright), Conex(copyright)) both of which operate up to but preferably below 400xc2x0 F. Applications operating at temperatures greater than 400xc2x0 F. and up to 550xc2x0 F. typically utilize fibers of glass, polyimide (P84) or PTFE (Teflon(copyright), Profilen(copyright), Rastex(copyright)). For this reason the selection of the substrate fiber or resin is in part determined by the conditions under which the filter will be used.
Second, the abrupt expansions from reverse pulse cleaning procedure places additional stress on the filter media. The stress results in cracking of the resin or stiffening agent, discussed below, thereby shortening the effective filter life. As the tensile and tear strength of the stiffening agent are lessened, the media tend to crack, split and/or tear, leading to the escape of unfiltered emissions through the tears. Thus, the combination of stresses due to the inward filtering flow and outward cleaning pulses reduce the effective filter life typically to six months or less when operated twenty four hours/seven days a week at a temperature of 350xc2x0 F. to 400xc2x0 F.
Conventional polymer filtering materials, including PPS based filtering material, has heretofore been treated with stiffening agent resin systems applied in a secondary process. The stiffening resins impregnate the substrate to strengthen and stabilize the filter, as well as aiding in pleating and pleat retention. These known stiffening resins include emulsions and/or dispersions of bisphenol based epoxies, acrylic based resins, melamine and phenol formaldehyde resins. These resins are commonly used in the textile industry and are recommended for use with textile products exposed to high temperatures. They are applied to impart the necessary features to allow the media to pleat and retain its form and shape at the desired application temperatures. The resins are typically applied via an impregnation process, where the media is totally immersed in a bath of the specified resin solution and then nip squeezed to remove the excess solution prior to drying. After drying the media is rendered stiff. The media is measured to ensure the correct level of resin has been applied, typically 10% to 25% add on to the basis weight of the base or substrate medium.
These conventional resins are not fully cross-linked or cured when initially dried onto the fabric. This allows the media to re-soften during subsequent high temperature processes, specifically the pleating process, where the softening allows the fabric to conform to the pleating action. After subsequent cooling the resin helps maintain the pleat structure. Unfortunately, upon initial exposure to elevated temperatures during filtering applications the media softens, and does not fully cure and re-stiffen for up to several hours. Once fully cured, the fabric is better able to withstand the rigors of reverse air pulsing, at low temperatures, without losing shape and form. However, pleat collapse or pinching can occur while the material is soft prior to curing. These resins that cure in two stages are called xe2x80x9cBxe2x80x9d staged resins. The initial drying and partial curing is commonly known as xe2x80x9cB-curingxe2x80x9d, followed by a complete or final curing.
While these stiffening agent resins render the treated filter fabric pleatable, conventional resins cannot withstand the mechanical wear and rigors of reverse air pulse cleaning, as these resins tend to crack. Commercially available filter media typically have a life expectancy, dependent on conditions, of about six months, due to the thermal and mechanical wear on the stiffening resin.
It is therefore desirable to provide a polymer stiffening agent capable of withstanding the temperature and cleaning pulses of high temperature industrial baghouses, and to provide a pleated filter medium utilizing the improved polymer stiffening agent
The present invention is directed to an improved polymer stiffening resin for use with a filtering medium. The stiffening resins of the present invention are polyimide (PI) based resins. Polyimides are a generic class of condensation polymers containing an xe2x80x98imidexe2x80x99 linkage as a linear or heterocyclic unit along the polymer backbone.
In a preferred embodiment the polyimide stiffening resins are applied to woven and non-woven polymer substrates to form pleated filter media. The non-woven substrates are calendered, and both types of substrates are immersion coated with the polyimide stiffening resin. The improved filter media of the present invention are especially well suited for use in applications at temperatures exceeding 300xc2x0 F., such as industrial baghouse filtering applications.
The present invention is also directed to a method of making the filter medium of the present invention. A preferred method comprises calendering a substrate; treating the calendered substrate with a polyimide stiffening agent; and curing the treated substrate.